Like nearly all mail servers, the one built into Mac OS X Server has two major components: an SMTP server (Postfix) that handles communication with other mail servers and a POP/IMAP server (Cyrus in Leopard; Dovecot in Snow Leopard) that handles retrieval and management of your users' mail. Depending on your needs, you might turn on only one or two of these protocols, but the most common arrangement is to offer all three. For each one, you have a variety of choices involving security.
In addition to SMTP, POP, and IMAP services, Mac OS X Server offers a webmail interface (SquirrelMail) and a mailing list server (Mailman), neither of which I cover in this book.
The first thing you should ponder is how you want to structure your mail services with respect to the other services you're providing with Mac OS X Server — and whether running your own mail server is the best solution for your organization.
My personal experience running mail services on Mac OS X Server has been that it's highly resource-intensive, which means that if your organization sends or receives a great deal of email — and especially if you employ server-based spam and virus filtering — it can slow down other processes running on the same computer (such as a web server). Your mileage may vary, of course, depending on such variables as the number and speed of your server's processors, the amount of RAM it has, the volume of mail transferred, and the variety of other processes ...